One of the fundamental properties of heart cells is their ability to alter their electrical characteristics in response to neuro-hormonal stimulation. This property allows the heart to adapt its physiological function to the range of activity it must meet. The work in this proposal focuses upon a delayed potassium channel in the sino atrial node and ventricle of the mammalian heart. This channel provides outward current (IK) that contributes to control of action potential duration, and thus changes in it will be critical to maintaining the proper relationship between systole (contraction) and diastole (filling time) as heart rate changes. In addition, because calcium ions enter heart cells during the period of depolarization of the action potential known as the plateau phase, changes in IK will indirectly affect calcium entry by controlling duration. In previous investigations, we have found that IK of the guinea pig ventricle is regulated by two enzyme systems (protein kinase A and C) and that this regulation has unique properties that distinguishes control of ventricular IK from that of the cardiac (L-type) calcium channel. The first aim of the present research proposal is to extend this work from guinea pig ventricle to the Sino Atrial (SA) node by characterizing the voltage dependence, channel selectivity, and single channel properties of the dominant SA node delayed rectifier in the guinea pig. The second aim is to test the hypothesis that IK channels are regulated differently by neurohormones in the ventricle and SA node. The third aim of the project is to characterize the functional properties of recombinant delayed rectifier K channels expressed in mammalian cells that have been transfected with the gene that encodes a unique, slowly activating potassium channel, minK. These experiments will use functional properties of the recombinant channels to provide further evidence identifying the protein underlying native IK channels. Then experiments will be designed to compare neurohormonal regulation of the recombinant and native channels and then to identify the molecular sites of action that underlie channel regulation. The results of this work will be of interest to the field of cardiac electrophysiology and to the more general area of neuromodulation of ion channel proteins.